Many insulated cables must meet requirements for strippability and roundness. Strippability (i.e., the ability to remove the jacket without damaging the conductors and insulation within the jacket) is particularly important when the cable is stripped by automatic machinery. Roundness is particularly important when a mold must be fitted around the cable as part of a process in which a terminal is formed on the end of the cable (sometimes referred to as "overmolding" and when the cable must pass through a precisely sized port, for example, prior to being sealed to the port. Roundness is usually expressed as a number obtained by dividing the minimum diameter by the maximum diameter and multiplying by 100. One example of a cable which must be both round and strippable is the twisted pair cable which is used in anti-lock braking systems (ABS) to connect the wheel speed sensors and the electronic controller; such an ABS cable preferably has a roundness of at least 96%.
It is not easy to make a cable which contains a small number of individual conductors (particularly a twisted pair of insulated conductors) and which is both round and strippable. A cable having good strippability can be made by a tube extrusion process, but the product has poor roundness. A cable having good roundness can be obtained by pressure-extruding a polymeric composition around two or more individually insulated conductors (and optionally filler rods of insulating material), but the jacket material tends to fill the interstices between the insulated conductors, making the jacket difficult to strip. The conventional solution to this problem is to apply talc powder or other release agent to the insulated conductors before applying the jacket. However, this does not prevent the jacket material from filling the interstices between the insulated conductors and is undesirable because of environmental concerns and potentially adverse effects of the release agent.
When an insulating jacket has been prepared by extrusion around a large cable, it often suffers from a problem referred to as "shrinkback", i.e. after an end portion of the jacket has been cut off in order to remove the jacket at a termination, the remaining jacket shrinks back from the termination. The tendency of a cable to shrink back can be assessed by cutting a short length of the cable, e.g. a 12 inch length, and maintaining it in an oven at an elevated temperature, e.g. 150.degree. C., for an extended time, e.g. 6 hours, and measuring the shrinkage of the jacket.
A shielded cable typically contains a metal braid or a wrapped metal foil which provides a continuous conductive shield over the insulated conductors. A cable with metal braid is difficult to strip because the braid is difficult to cut. Metal foil, when wrapped directly over the insulated conductors, is difficult to strip without damage to the insulated conductors, because it conforms to the shape of the insulated conductors as it is wrapped.